Material Selection for SOx/NOx Scrubbers: Why PE Roto-Lined Steel Offers Superior Corrosion Resistance and Cost Efficiency

Sulfur and Nitrogen Oxides pollution abatement is continuing to be as critical as ever in the context of tighter environmental emission norms and increasing concerns of global warming. The process chemistry and unit operations associated with SOx and NOx removal techniques are standard and mature. One of the major challenges associated with the technologies is the selection of appropriate materials of construction that provide the lowest total cost of ownership and a high level of corrosion resistance. The advent of newer materials and fabrication techniques have increased the material options available. This paper takes a closer look at one of the options: polyethylene (PE) lined steel pipes manufactured by roto-lining process. This material, though not a recent invention, is relatively new to the field of SOx/NOx scrubbing systems and is a strong contender among the available options.

SOx / NOx Scrubbers

SOx and NOx abatement applications are encountered in coal and oil-fired power plants, steam boilers and marine engines. Though all these applications have several common features, this paper focuses on marine applications, particularly scrubbers installed in ships.

A typical open loop scrubber used for installation in ships is shown in Fig. 1. The simplest scrubber system available is the open loop scrubber, where water is sourced from the surrounding sea, pumped through a filter and sprayed into the scrubber using nozzles that disperse water into droplets. An open loop scrubber is efficient only if the source of water is alkaline. This can either be done by adding an alkali chemical or by utilizing seawater, which has a natural alkalinity derived from the bicarbonate ion (HCO₃^-) present in seawater.

The hot exhaust gas from a ship’s engine is led to the inlet at the bottom of a cylindrical packed scrubber to encounter a counter-current flow of aerated sea water sprayed from the top of the tower, wherein the SOx and NOx in the exhaust gas is absorbed by the sea water and oxidized by oxygen in the aerated sea water to sulfuric acid.

The water is discharged back into the sea after particulate matters are removed. The type of solid waste generated from scrubbing is classified as hazardous waste, and therefore stored and later disposed to land-based disposal facilities. Closed loop scrubber systems are developed for no-discharge zones; this requires that the scrubber system works on a recirculated flow that is later discharged as the vessel leaves a no-discharge area. Depending on the amount of water consumed and the size of the buffer tank, the closed loop system can only run for a certain amount of time until the liquid is saturated. Therefore, to avoid saturation and increase the time span, the liquid phase is continuously bled off, adding more seawater into the system.

Scrubber System – Chemistry and Corrosive Conditions

SOx are formed by a ship’s engines during the combustion process due to the oxidation of naturally occurring sulphur in the fuel. In the exhaust gas, the major part of SOx is present as SO₂, and a minor portion is SO₃.

Reactions involving SO₂:
Gaseous SO₂ dissolves in seawater and subsequently is ionized, generating bisulphite and sulphite ions.

SO2 + H2O   ⇌   H+ + HSO3-
HSO3-      ⇌   H+ + SO3^2-
SO3^2- + 0.5 O2 → SO4^2-
  

The generated hydrogen ions are neutralized by the alkalinity of seawater (mainly due to its bicarbonate content), thereby consuming alkalinity.

Reactions involving SO₃:
Gaseous SO₃ dissolves in water, forming sulfuric acid, which dissociates to sulphate.

SO3 + H2O → H2SO4 (Sulfuric Acid)
H2SO4 → H+ + HSO4- → 2H+ + SO4^2- (Sulfate)
  

The generated hydrogen ions are neutralized by the alkalinity of seawater, thereby consuming alkalinity.

Where no effluent discharge is permitted, a closed loop scrubbing process with normal water and external alkali dosing can be employed. The Arctic, Northwest Pacific, and Norwegian ocean are some of the areas where alkalinity of seawater is inadequate for open loop scrubbing. In the interest of brevity, a detailed description of the closed loop process is not included here. The corrosive conditions encountered in the open loop process are more aggressive than those in the closed loop process (seawater is more corrosive than fresh water), hence the open loop process covered in this paper may be considered a worst-case scenario.

The surface pH of seawater usually ranges from 8.1 to 8.9. During scrubbing, the hydrogen ions generated are neutralized by the alkalinity of seawater, resulting in a pH of 4 to 5. The corresponding concentration of sulfuric acid can be read off the chart in Fig. 2.

The absorption of NOx gases into the liquid phase is more complex than that of SO₂. Not only is the gas-phase NOx in equilibrium with the liquid phase, but equilibrium and kinetic reactions also occur in the gas‑phase. This complicates the absorption chemistry of NOx, since different nitrogenous compounds are formed in the gas phase. These compounds can also be dissolved into the liquid phase and lead to different possible reactions in the liquid phase. NOx, in the form of NO or NO₂, can be absorbed into the liquid phase.

NO(g)   ⇌   NO · H2O
NO      ⇌   [NO · H2O]/pNO

NO2(g)  ⇌   NO2 · H2O
HNO2    ⇌   [NO2 · H2O]/pNO2

2NO2 + H2O → HNO2 + HNO3
HNO2    ⇌   H+ + NO2-
HNO3    ⇌   H+ + NO3-
  

Presently ship vent gas scrubbing systems are designed mostly for SOx removal only. However, technologies such as the CSNOX Wet Scrubbing Technology developed by Ecospec, Singapore for removing SOx, CO₂ and NOx in a single wet scrubber will put ships using such technologies at a distinct advantage in the future when marine emission regulations may be extended to NOx emissions as well.

Selection of Materials of Construction

From the preceding description of the chemistry involved in the scrubbing process, the materials of the various components of the scrubber system should be suitable for the respective media and temperature indicated in Table 1 below.

Table 1 – Material of Construction Options for SOx and NOx Scrubbers

Ɨ Non-metallic materials like PVDF/PFA lined Steel are potential options that have superior corrosion resistance and temperature resistance as compared to PE and PP lined steel and may be potential substitutes for Duplex SS.
** Vinyl Ester FRP

PE Lining by Rotational Lining (Roto‑lining)

The rotational lining is a process of lining the inside of pipes or other SOx components with a seamless, one‑piece inner layer of PE plastic. In this lining technique, the lined spool is produced by heating and rotating a carbon steel spool with a polymer (in granular/powder form) placed inside the pipe spool. The polymer melts and forms a seamless and uniform liner on the internal surface of the carbon steel pipe. The engineered polymers used for the rotational lining process include additives which increase the bond and adhesion with the prepared metal substrate.

The choice of polymer depends on the required chemical resistance properties. Polyethylene, polypropylene, or other polymers are used for roto‑lining. Polyethylene is the workhorse of the segment and accounts for over 80% of roto‑lining applications. The lining thickness varies from 2 mm to 8 mm. The heavy lining thickness allows post‑machining of critical surfaces that would not be possible with a thinner lining applied by other methods. Virtually any type of metal weldment or casting may be roto‑lined. Typical items include tanks, carbon steel pipes, fittings, and complex welded structures.

Advantages of Rotational Lining

• Costs for lining and tooling are relatively low.
• The roto‑lining technique is easily adapted to short production runs, particularly when sets of multiple‑cavity molds are used.
• Hollow, totally enclosed items, as well as pieces with openings, can be made.
• Rotational lining eliminates the need for secondary tooling.
• There is little or no waste due to resin scrap.
• Wall thickness and piece weight can be easily controlled.
• Rotational lining procedures ensure uniform wall thickness; deviations can be controlled within a tolerance of ±10%.
• Pieces with intricate contours and undercuts can be easily molded.
• Virtually any size piece can be rotationally molded.
• There is a minimum of cross‑sectional deformation and warpage.
• Rotational lining yields pieces with excellent surface detail and finish.
• Rotationally lined items are virtually stress free.
• Identical or similar items, or different sections on one piece, can be molded at the same time in different colors on a single spindle.
• Plastic or metal inserts can often be lined as integral parts of the item.
• Double wall constructions are feasible.

Rotational lining is a mature technology proven in various harsh chemical applications. Figures 3 to 5 demonstrate the diverse capabilities: from large tanks to complex shaped piping.

In addition to a wide range of sizes and intricate geometries, material options within polyethylene (LDPE, LLDPE, LMDPE, HDPE, and cross‑linkable polyethylene XLPE/XLDPE) broaden the application range of PE lined equipment.

PE Lining Material Options – Cross‑Linked Polyethylene (XLPE)

XLPE is a form of polyethylene with cross‑links. Molecules arrange in a specific pattern with actual chemical bonds formed between individual molecules, locking them into a semi‑rigid three‑dimensional shape. Heating does not weaken these bonds, so XLPE retains its physical properties at elevated temperatures. Primary advantages: improved creep, low‑temperature impact strength, heat resistance, improved resistance to stress cracking, and reduced permeation. These attributes allow XLPE to be used in demanding applications such as fuel and chemical tanks.

Key factors of XLPE for SOx/NOx scrubbers:

• Elevated temperature capability (up to 70 °C)
• Higher resistance to oxidative media like sulfuric acid and sodium hypochlorite (encountered in ballast water treatment systems)
• Excellent stress corrosion cracking resistance
• Excellent impact strength

PE has a clear edge over competing materials like VE‑FRP, as summarized in Table 2. Metallic options (Duplex SS, Titanium) are more expensive and excluded from this direct comparison.

Table 2 – Comparative Features of XLPE Lined Steel vs. VE‑FRP

ƗƗ – Reduction in mechanical properties by water/chemical absorption has been attributed to debonding of the fiber‑matrix interface, leading to delamination, cracking, and plasticizing of the matrix (5).

Based on the above comparative evaluation, suggested PE lined materials for various parts of a SOx/NOx scrubber system are shown in Fig. 8.

Conclusion

PE roto‑lined equipment and piping offer a robust and cost‑effective solution for material selection in SOx/NOx scrubber systems. It is worthwhile to make a detailed evaluation of this option before final material choice, as it has several technical advantages over competing materials and could potentially provide the lowest total cost of ownership.

References

1. Yao, G. Ziegmann, “Water Absorption Behavior and Its Influence on Properties of GRP Pipe”, Journal of Composite Materials, Vol. 41, No. 8/2007.
2. Referenced pH‑concentration chart (source as in original paper).
3. Rotational lining advantages – industry standard data.
4. Cross‑linked polyethylene properties – industry literature.
5. Yao, G. Ziegmann as above.